Intravascular interventions may be supported by magnetic resonance imaging. This requires that a medical instrument introduced into the body is visualized in order to be able to steer the medical instrument precisely through the body. The automatic detection of the instrument and its visualization or the adaptation of the image planes that are represented to the moving instrument is known as “tracking”. Examples of such instruments are catheters or guide wires for catheters.
A distinction is made between active and passive tracking. Passive tracking is based on image artifacts or image properties that are caused by the instrument.
Active tracking requires an apparatus that may receive a position-dependent signal. A signal of this type may be generated by a magnetic resonance pulse sequence or by separate signal generators. The signal may be detected directly.
The detection may take place indirectly via magnetic resonance. Herein, initially atomic nuclei are resonantly excited to the Larmor frequency, which is the frequency at which the spins of the atomic nuclei precess about the direction of the outer magnetic field. The atomic nuclei emit the excitation energy by radiating an electromagnetic wave that also has the Larmor frequency. If this wave has positional information, it may be utilized for active tracking by detection of the wave.
Suitable apparatuses that enable active tracking may be external high frequency antenna units, also referred to below as HF coils, which are able to receive electromagnetic HF signals by induction. Tracking coils of this type may have a small size and may be arranged on a catheter or similar devices. The illumination field of the tracking coil may also be very small, e.g., the spatial interaction region around the external HF coil from which HF signals may be received by the external HF coil.
Tracking information may also be detected in sequences for anatomical imaging. The image acquisition of a magnetic resonance tomograph is a relatively slow modality, in particular as far as high resolution image acquisition is concerned. For precise positioning of a catheter or another device, however, it is precisely a high degree of resolution at the target location that may be needed.
Depending on the tracking type, it is possible to acquire tracking information on the position of the device or the marker with sequences significantly quicker than the sequences for an anatomical image. Such sequences, however, do not provide any, or only too coarse, representations of the anatomy so that no exact position is possible by these sequences.